Vehicle transmissions typically include a system of planetary gear sets and hydraulically-actuated clutches. A hydraulic control unit (HCU) controls the supply of hydraulic fluid (e.g., oil) to the various clutches to control a gear (park/neutral/drive) and a drive gear ratio of the transmission. Some engines have start/stop functionality where the engine is temporarily shut off when the vehicle is stopped. The transmission, however, requires hydraulic pressure to immediately shift the transmission into drive upon restart of the engine. Conventional transmission control systems utilize an electric oil pump, which is operable when the engine is off, or a hydraulic fluid accumulator to supply this immediately required hydraulic pressure. Both of these components, however, increase vehicle costs and/or packaging size requirements for the transmission. Accordingly, while these systems work well for their intended purpose, there remains a need for improvement in the relevant art.